Evaluation of Microhardness of Two Bulk-fill Composite Resins Compared to a Conventional Composite Resin on surface and in Different Depths

Statement of the Problem: One of the problems with light-cured composite resins is the limitation and inadequate depth of curing and polymerization, resulting in low surface microhardness and restoration failure. Purpose: The present study aimed to compare the surface microhardness of two different bulk-fill composite resins and one conventional composite resin using the Vickers microhardness test. Materials and Method: In the present in vitro study, 108 samples from two different bulk-fill composite resins (Tetric N Ceram and Xtrafil) and one conventional composite resin (Filtek Z250) were prepared in metallic molds (2×4×10 mm) (n=36 for each composite resin). Six samples from each composite resin (n=6) underwent a hardness measurement test at specific depths (0.1, 1, 2, 3, 4 and 5mm). The samples were then stored at 37ºC for 24 hours, followed by a microhardness test at the depths mentioned above. Results: In all the composite resin samples, microhardness decreased with an increase in depth. The highest microhardness was recorded in Filtek Z250, followed by Xtrafil, with no significant difference. The lowest microhardness was recorded in Tetric N Ceram bulk-fill. Both bulk-fill composite resins at all the depths exhibited depth-to-surface standard microhardness (>80%). Conclusion: According to the results, both evaluated bulk-fill composite resins exhibited favorable surface microhardness up to a depth of 5 mm.


Introduction
Composite resins are widely used in dentistry. One of the concerns with the use of light-cured composite resins is their limited light penetration, resulting in inadequate curing depth and polymerization [1].
Adequate polymerization is a vital factor in achieving favorable mechanical properties, which ensures satisfactory clinical efficacy of composite resin restora- The release of uncured monomers, followed by decreased biocompatibility of restorations and compro-mised physical properties, including low color stability, result from inadequate polymerization of composite resin restorations [3].
Various factors affect the photo-polymerization of composite resins, including composite resin type, its color and translucency, the thickness of each composite resin layer, the distance between the tip of the lightconducting nozzle and the composite resin surface, the type of the light-curing unit, curing parameters, irradiation mode, photo initiators, the size and distribution of fillers, and viscosity [4][5].
Composite resins can be cured at specific depths, depending on the penetration of light into the composite resin bulk. The light energy of light-curing unit decreases gradually as it transverses through the composite resin bulk [6]. Several techniques have been proposed to overcome this problem, including the layering technique for composite resin placement or the adjustment of the irradiation mechanisms. However, the layering technique is time-consuming, with a high risk of air bubble entrapment and contamination [7].
Bulk-fill composite resins were introduced to overcome the problems above. According to the manufacturer, these composite resins, which have become very popular with dentists due to their ease of use, have a 4mm curing depth. Besides, this bulk of composite resin can be cured in one stage because it has low polymerization shrinkage and minimal polymerization stress [8].
In bulk-fill composite resins, high translucency, high monomer technology, modifications in fillers and use of new photo initiators have decreased polymerization stresses and increased the curing depth, with a significant adaptation with the cavity walls as an advantage.
Surface hardness is used to predict materials' wear resistance to abrasion and abrasive caused by opposing teeth. The depth-to-surface microhardness ratio of composite resins is 0.8-0.85, so that it can be ensured that the base has adequately been polymerized [11].
Microhardness is defined as the resistance against penetration or permanent indentation of the surface, which is a criterion for resistance against plastic deformation and is calculated by dividing the force by the indented surface area. Vickers test is one of the most common tests in this respect [11][12]. The evaluation of the curing depth of composite resins by measuring the hardness is imperative since there is a relationship between an increase in hardness and curing depth [12].
Although the clinical use of bulk-fill composite resins is increasing, several previous studies have not fully confirmed their mechanical properties [8,13]. Therefore, it is necessary to evaluate these properties in composite resins, including degree of conversion (DC) or surface hardness, especially in the long term [10,14].
Since there is a lack of adequate data in this filed, and considering the discrepancies about adequate curing depth and durability of bulk-fill composite resins in the use of these composite resins with >2 mm depths, the present study aimed to compare the microhardness of two different types of bulk-fill composite resins and one conventional composite resin at different depths by using Vickers microhardness test. The null hypothesis was that all composites have the same Vickers microhardness in different evaluated depths.

Materials and Method
In the present in vitro study, samples of Tetric N Ceram bulk-fill and Xtrafil bulk-fill and Filtek Z250 conventional composite resins were fabricated using bronze molds. Table 1 presents the characteristics of the composite resins evaluated.

Statistical Analysis
The data were analyzed with SPSS 25.0. Since data were distributed normally, Two-way ANOVA was used to compare microhardness changes between the groups.
In addition, post hoc Tukey tests were used for two-bytwo comparisons of groups. The acceptable type I error was set at 0.05 in this study (α=0.05).   Xtrafil composite resin [21][22]. In addition, the multihybrid filler technology has been used in this material, resulting in a decrease in polymerization shrinkage, a 4mm curing depth, high surface hardness, and high resistance to abrasion in this material [23]. The high hardness of the Z250 composite rein might be attributed to its fillers composed of quartz and ceramic particles [24].

Results
The significantly low surface hardness of Tetric N Ceram bulk-fill composite rein might be attributed to the initiator/catalyst system, the type of monomer, and the use of barium glass fillers with re-polymerized particles in the structure of this composite resin [22]. The parameters affecting microhardness include the shape and distribution of fillers, the shape and density of particles, and the type and concentration of the monomer, which are different in different composite resins [25].
In all the composite reins evaluated, the surface hardness was significantly higher than that of the depth.
Hardness decreased with an increase in thickness, indicating that the surface of composite rein depends on the light intensity at a lower rate because it absorbs the nec-essary radiation energy due to its vicinity to the lightconducting tip [26][27]. In the present study, in all the composite resins evaluated, the sample's depth-tosurface microhardness ratio was >80% (almost 88%).
Al-Mansour et al. [28] reported that the proper curing depth in Tetric N Ceram bulk-fill composite resin is due to Ivocerin in its structure, which is an initiator with a germanium base. According to the manufacturer, it has a higher curing activity than camphorquinone because generates at least two free radicals for polymerization initialization compared to camphorquinone, the most widely used visible-light photo-initiator in RBCs that generates only one radical [1,29]. Besides, it can initiate polymerization without adding amine by creating two radicals, which is more effective than the camphorquinone system with only one radical. Other studies have shown that despite a high filler content in Tetric N Ceram bulk-fill composite, the depth-to-surface microhardness ratio <80%, which is different from the present study [22,30].  [32][33]. Any discrepancy between the unit's radiation wavelength and the photoinitiator's sensitivity might give rise to limitations in the creation of free radicals and a disturbance in the polymerization process [34].
The employment of new resins and modified regulators and fillers has increased the curing depth of bulkfill composite resins. Besides, the amount of light penetrating the composite resin depends on the amount of the light reflected, scattered, and absorbed and all of these factors depend on the composite resin structure.
Composite resins with smaller fillers scatter more light [35].
Evidence indicates that an increase in filler content decreases translucency due to an increase in light reflection at filler-resin interface. Translucency increases with an increase in filler size. Therefore, the size, radiopacity, translucency, and pigments or filler particles affect the passage of light through the material, which in turn affects the curing depth [36].
Further studies are suggested to evaluate other bulkfill composite resins, abrasion resistance, and fracture resistance.
This study was in vitro, therefore it may be different from the clinical situation and besides that, there are some factors available in the mouth like saliva, enzymes, different food and beverages with different pH and temperature that affect composite microhardness during the time.

Conclusion
Under the limitations of the present study, it was concluded that microhardness decreased in all the composite resins with an increase in depth. In both bulk-fill composite resins, the depth-to-surface microhardness ratio in all depth was at a standard level (>80%).

Conflicts of Interest
None declared.